This invention relates to a method of preparing high viscosity cellulose ethers.
Cellulose ethers such as methylcellulose and hydroxypropyl methylcellulose are used as thickeners in such diverse applications as water-based paints, foods and cosmetics. Presently, the 2 percent aqueous solution viscosities for these products range from below 5 to a maximum of about 100,000 cps, with the exact value being a function of the molecular weight of the cellulose polymer backbone in the material. In general, the greater the molecular weight of the cellulose polymer the larger the intrinsic viscosity of the cellulose ether product. Both wood and cotton linter celluloses are used to prepare cellulose ethers, with economics favoring the use of wood pulp. The more expensive cotton linters have been required in the production of high viscosity products in the 100,000 cps range.
Cellulose ether products with intrinsic viscosities greater than 100,000 cps would be desirable for they would provide more efficient thickening with lower raw material usage and reduced cost. Production of such high viscosity cellulose ethers has not been possible, however, due to the chemical and mechanical degradation of the cellulose polymer backbone that occurs during the manufacturing process. For example, the presence of an oxidizing agent such as air during the alkalinization and etherification steps of the process significantly reduces the viscosity of the product. Viscosity is also lost if excess caustic is not neutralized before the product is extracted and dried. Even if care is taken to exclude oxidizing agents and to neutralize excess caustic before work up, there is still a significant viscosity loss when the material is ground in a ball mill, hammer mill or disk mill to give finished product. Therefore, because significant viscosity loss occurs when the cellulose ether is ground, it is difficult to prepare a product which has small particle size and high viscosity.
It would be desirable to have a process for preparing a cellulose ether product which has small particle size and there is minimal degradation of the cellulose polymer backbone. Thus, the resulting cellulose ether products would have higher intrinsic viscosities.